Pyrolysis heating rate effects

Figure 7. Effects of pressure and coal particle size on yields of total volatiles, tar plus hydrocarbon liquids, all hydrocarbon gases, and methane, from bituminous coal pyrolysis. Heating rate = 1000°C/sec. Temperature = 1000°C. Isothermal holding time = 2-10 sec. Particle diameters, ixm C) 74 (X) 297-833 (O) 833-991 (14).

Kerogen Decomposition. The thermal decomposition of oil shale, ie, pyrolysis or retorting, yields Hquid, gaseous, and soHd products. The amounts of oil, gas, and coke which ultimately are formed depend on the heating rate of the oil shale and the temperature—time history of the Hberated oil. There is Htde effect of shale richness on these relative product yields under fixed pyrolysis conditions, as is shown in Table 5 (10). 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

In some cases, several of these processes occur simultaneously, depending on the sample size, the heating rate, the pyrolysis temperature, the environment, and the presence of any additives. Although polymer degradation schemes can be greatly altered by the presence of comonomers, side-chain substituents, and other chemical constituent factors, the ultimate thermal stability is determined by the relative strengths of the main-chain bonds. Many additives and comonomers employed as flame retardants are thermally labile and as a result the thermal stability of the polymer system is reduced. In order to reduce the observed effects of the flame-retardant additives on the thermal stability of the polymeric materials, more thermally stable and hence inherently flame-resistant polymers are of increasing interest. 

Cone calorimetry according to the ASTM E1354138 or ISO 5660139 standards are commonly used in the laboratory to screen flammability of materials by measuring heat release characteristics of the compound.116140 This device is similar to FPA but does not have the versatility of FPA. The cone calorimeter can determine the ignitability, heat release rates, effective heat of combustion, visible smoke, and C02 and CO development of cable materials. This test has been used extensively for wire and cable material evaluation. The microscale combustion calorimeter (MCC), also known as pyrolysis combustion flow calorimeter (PCFC), was recently introduced to the industry for screening heat release characteristics of FR materials.141142 This device only requires milligram quantities of test specimen to measure the heat release capacity (maximum heat release potential). Cone calorimetry and MCC have been used in product development for flammability screening of wire and cable compounds.118... 

These points respectively define the temperature at which, on reheating, a cooled or quenched pyrolysis residue begins to soften and develop fluidity and the temperature at which the liquid residue begins to change its chemical composition, i.e., when pyrolysis recommences. This latter point is more ambiguous since it is determined from a thermogravimetric experiment which is subject to the effects of experimental procedure, as discussed earlier. Therefore, it is essential to use a fixed heating rate, etc., in the determination of the decomposition temperature. The volatile content can be determined in the same experiment. 

Because pyrolysis reactions do not occur at sharply defined temperatures, the heating rate has a marked effect on the nature and distribution of pyrolysis products, as summarized in Table 19.14. Solomon and coworkers conducted extensive work on the kinetics of coal devolatilization, and many reviews are available.36... 

Extraction rates in dependance of temperature and pressure are very different and are shown in figure 1. At 50°C the extraction efficiency is very low. Between 60°C and 70°C extraction is more effective and depends little on the carbon dioxide flow rate. For a given temperature the variation of pressure has only a small influence on the extraction rate, whereas the variation of temperature at a fixed pressure leads to great differences in extraction efficiency. The parts treated at 50°C often show cracks or bubbles. At 75°C they soften and deform. The optimal temperature range for this binder system is 60-70°C. Here it is possible to extract up to 70 vol.% of the binder without damaging the workpieces. As a result of the reached porosity it is possible to reduce the time of the furthermore necessary pyrolysis of the remaining binder components drastically (heating rate 10°C/min from r.t. up to 1000°C). The dimensions of the examined parts were 4 x 5 x 60 mm. 

Pyrolysis. All of the Texas lignite pyrolysis data reported in the literature have been based on slow pyrolysis rates (e.g., 3-10 C/min). Goodman et al. (20) performed an early study (1958> on the effect of final carbonization temperature on the yields from several lignites, one of which was a Wilcox lignite. More recently Edgar et al. (21) reported a series of atmospheric pressure studies to evaluate the effects of other carbonization parameters (heating rate, particle size) on carbonization yield and decomposition rate. 

The effect of hydrogen on coal pyrolysis can further be illustrated by Figure 4, where we compared derivative thermograms of Wyoming coal pyrolyzed at 200 psig of N2 and 200 psig of H2 at the same heating rates (20 C/min). The secondary hydropyrolysis peak... 

TIjffective use of the vast coal reserves of the United States requires quantitative information on their conversion behavior in commercially important reactions. Studies using small-scale equipment and aimed at determining the effects of temperature, pressure, particle size, and heating rate on the rapid thermal decomposition of coal in atmospheres of both helium (pyrolysis) and hydrogen (hydropyrolysis) have been in... 

The studied variables were the heating rate and the final pyrolysis temperature. Their effects on the product yields, gas composition and energy recovery were analysed. The effect of the two cited variables on the specific surface area of the resulting char is also analysed. 

Table I Effect of the heating rate (Vi°) in pyrolysis of hydrolysis lignin (T 00 C).

In wood pyrolysis, it is known that several parameters influence the yield of pyrolytic oil and its composition. Among these parameters, wood composition, heating rate, pressure, moisture content, presence of catalyst, particle size and combined effects of these variables are known to be important. The thermal degradation of wood starts with free water evaporation. This endothermic process takes place at 120 to 150 C, followed by several exothermic reactions at 200 to 250°C, 280 to 320 C, and around 400 C, corresponding to the thermal degradation of hemicelluloses, cellulose, and lignin respectively. In addition to the extractives, the biomass pyrolytic liquid product represents a proportional combination of pyrolysates from cellulose, hemicelluloses. 

Moisture content of wood appears to be the most important physical parameter to take into account in wood carbonization. An increasing moisture content decreases the production of pyrolytic liquids and increases the production of non condensable gases by enhancing secondary reactions of pyrolysis inside the solid matrix. This effect is increased by the shape of wood samples wet blocks of 4 4 16 cm produce less pyrolytic liquids and more non condensable gases than other wood samples. However, moisture content does not influence the chemical composition of carbonization products. This confirms the fact that water present in wood acts physically and not chemically in the carbonization of wood at low temperature and heating rate. 

An interesting comparison regarding the effect of different substituents on pyrolysis results for a polymer can be obtained from the Py-GC/MS analysis of a poly(2-vinylpyridine) and that of a poly(4-vinylpyridine) sample. The sample of poly(2-vinylpyridine) has a M = 5,000. The pyrolysis was done for both samples from 0.4 mg material, at 600° C in He, at a heating rate of 20° C/ms with 10 s THT, and with the separation on a Carbowax column (see Table 4.2.2). The pyrogram for poly(2-vinylpyridine) is shown in Figure 6.5.11 with the peak identification in Table 6.5.11 (see Figure 6.5.13 for the pyrogram of poly(4-vinylpyridine)). 

A large volume of work has been reported on rapid devolatilization of coal (heating rates approximating process conditions (21,22). Recently, the effects of coal minerals on the rapid pyrolysis of a bituminous coal were reported by Franklin, et al ( 23). They found that only the calcium minerals affected the pyrolysis products. Addition of CaCO3 reduced the tar, hydrocarbon gas and liquid yields by 20-30%. The calcium minerals also altered the oxygen release mechanism from the coal. Franklin, et al. attribute these effects to CaCOj reduction to CaO, which acts as a solid base catalyst for a keto-enol isomerization reaction that produces the observed CO and H2O. 

The 1-alkene/n-alkane ratios in the oil, measured by capillary-column gas chromatography/mass spectroscopy, also increase with the addition of inert diluent (Figure 6). This effect and the previously demonstrated dependence on heating rate are consistent with a free-radical mechanism. In addition, we noted that alkene/alkane ratios for even-numbered hydrocarbons are significantly higher the ratios for odd-numbered ones. We do not understand this effect at this time but suspect that it is related to the structure of kerogen and the mechanism of its pyrolysis. 

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